The present invention relates generally to a novel prestretched film for packaging and stretch wrapping, produced by a unique method with a novel apparatus.
The construction and properties of films, and methods and processes for making the same are well known in the relevant art, as is indicated by the following United States patents, for example
______________________________________ Alles 2,767,435 10/23/56 Holladay et al. 3,244,680 04/05/66 Hufnagel et al. 3,351,697 11/07/67 Colombo 4,456,660 06/26/84 Chill 3,619,460 11/09/71 Stockmeyer 4,676,943 06/30/87 Thimon 4,767,578 08/30/88 ______________________________________
Stretch wrap films are used quite extensively in the modern world. Specifically, stretch wraps are used to secure separate unitary packages together to form a unit. The stretch wraps can also attach the unit to a support member, such as a pallet and the like. Thus, stretch wraps are commonly used in shipping articles from one place to another. The stretch wraps hold the individual packages together to reduce their shifting during transport as well as to assist in preventing damage to them and their contents. Accordingly, stretch wraps are used extensively in transporting packages from a manufacturer to a warehouse, stores, and to end consumers, for instance. To function properly, these films should be stretchable to tightly hold the packages, and also should have a resiliency sufficient for constricting around and conforming to an external configuration of the unit formed by the packages. However, some of these films constrict so tightly around the external configuration as to damage the packages and their contents. Additionally, these films are often composed of polyethylene, polyvinyl chloride, ethylene vinyl acetate, ethylene methyl acetate, and ethylene copolymer with higher alpha olefins, commonly referred to generally as linear low density polyethylene (hereinafter "LLDPE") films. These compounds are somewhat costly to manufacture.
In an effort to solve a portion of the cost problem, some film producers have reduced the gauge or thickness of the films, thereby reducing somewhat the amount of material required for a particular packaging situation. Heretofore, this has been done by stretching or drawing the plastic while it is still in the molten state at a relatively high temperature as it exits from the extruder. Some persons have even stated that stretch wrap films having a reduced thickness are stronger than thicker films.
These statements, however, are not precisely correct. The more correct statement is that a decrease in film thickness almost invariably results in a significant decrease in absolute strength, hereinafter referred to as "AS" (units of breakage force per units of width), while the oft-reported relative tensile strength, hereinafter referred to as "RTS" (units of breakage force per unit of cross sectional area), may remain constant, or even increase. This does not result in a stronger film, but is a simple misinterpretation of the underlying mathematics. For example, if the thickness of the film is reduced by 50 per cent, and the absolute strength decreases by 40 per cent, then the relative tensile strength increases by 20 per cent. Specifically, if it takes 100 pounds of force to break a 20 inch wide by 1.0 mil thick film, then: EQU AS=100 lbs/20 in=5.0 lbs/in.
RTS=100 lbs/(20 in.times.0.001 in)=5000 lbs/square in.
And, if it takes 60 pounds of force to break a 20 inch wide by 0.5 mil thick film, then: EQU AS=60 lbs/20 in=3.0 lbs/in
and, EQU RTS=60 lbs/(20 in.times.0.0005 in)=6000 lbs/square in.
Thus, the 50 per cent downgauging has resulted in a weaker film because AS is significantly lower in the downgauged film. An unsuspecting person, by simply looking at the numbers without understanding the underlying mathematics, might believe that the downgauged film is stronger. RTS is not a reliable indication of film strength because the thickness of the film appears in the denominator, making any reduction in thickness appear to create a stronger film.
The prestretched film, and the unique apparatus and method for producing the same, according to the teachings of the present invention, is intended to assist in solving some of the above-detailed problems, among others, while still providing a viable product.